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Review
, 23, 8-13

Integrated Circuits: How Transcriptional Silencing and Counter-Silencing Facilitate Bacterial Evolution

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Review

Integrated Circuits: How Transcriptional Silencing and Counter-Silencing Facilitate Bacterial Evolution

W Ryan Will et al. Curr Opin Microbiol.

Abstract

Horizontal gene transfer is a major contributor to bacterial evolution and diversity. For a bacterial cell to utilize newly-acquired traits such as virulence and antibiotic resistance, new genes must be integrated into the existing regulatory circuitry to allow appropriate expression. Xenogeneic silencing of horizontally-acquired genes by H-NS or other nucleoid-associated proteins avoids adventitious expression and can be relieved by other DNA-binding counter-silencing proteins in an environmentally-responsive and physiologically-responsive manner. Biochemical and genetic analyses have recently demonstrated that counter-silencing can occur at a variety of promoter architectures, in contrast to classical transcriptional activation. Disruption of H-NS nucleoprotein filaments by DNA bending is a suggested mechanism by which silencing can be relieved. This review discusses recent advances in our understanding of the mechanisms and importance of xenogeneic silencing and counter-silencing in the successful integration of horizontally-acquired genes into regulatory networks.

Figures

Figure 1
Figure 1. Architectural and regulatory logic of the PhoP regulon
The PhoP regulon contains both ancestral and horizontally-acquired genes, a few of which are depicted here. Promoter architecture, i.e. the position and orientation of a transcription factor binding site (green arrows) relative to the transcription start site (TSS; bent arrow) of horizontally-acquired genes, is variable. Horizontally-acquired genes lacking conserved promoter architecture are not directly activated, as they do not permit an appropriate interaction between PhoP and RNAP. Genes exhibiting ancestral architecture are directly activated by PhoP, including those that have been horizontally-acquired (orgB). Blue boxes indicate the approximate positions of the −10 and −35 boxes. The figure is based on results presented elsewhere [5, 41, 42].
Figure 2
Figure 2. Mechanisms of activation and counter-silencing
Ancestral core genes are transcriptionally inactive (OFF) until a transcription activator (green ovals) binds at a conserved position (green arrows) relative to the transcription start site (TSS; bent arrow) and interacts directly with RNAP (yellow) to promote transcription (ON). In contrast, horizontally-acquired genes are repressed by oligomerized H-NS (purple ovals), which binds a large region of promoter DNA and occludes RNAP binding. A counter-silencing transcription factor can bind at an alternative site relative to the TSS to re-model the H-NS-DNA complex, thereby restoring RNAP binding and transcription. Blue boxes indicate the approximate positions of the −10 and −35 boxes at each promoter.

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